Purpose: In prone breast treatments a carbon fiber support device resides under the contralateral breast. Tangent beams are designed to encompass the treated breast and these often pass through the board at a shallow angle, resulting in significant attenuation. Our planners account for this attenuation by added field-in-field dose to the deep part of the breast, through the board. Concern was raised about how accurate the treatment delivery is when the inherent uncertainties of patient position are accounted for. Further, transmission measurements are usually carried out perpendicular to the board; a non-clinical situation. The goal of this study is to evaluate the dosimetric effect of board and the robustness of the plan to positional uncertainty.
Methods: Twenty two breast patients treated on a commercial prone breast board between 2017 and 2020 were selected for this retrospective study. To evaluate the board's attenuation we compared the plans with the board removed from the dose calculation. To quantify the robustness of this technique we moved the beam isocenter wrt the patient and board.
Results: When the breast board is removed from a plan which was designed to account for the board attenuation, the average point dose increases by 7.48%, with a maximum of 22%. Comparing results with a mixed ANOVA and least square means analysis, our robustness evaluation indicates that anterior shifts at every magnitude (1mm through 5mm) make a significant difference in all dose statistics (D95, max, 95% prescription coverage and homogeneity index) investigated. Right/left shifts resulted in an insignificant change in dose statistics.
Conclusion: Prone breast boards can add significant dosimetric uncertainties into the treatment delivery process. Accounting for plan robustness in the design of the plan is highly recommended. Prone breast board design, with supports moved away from the beam-path, is warranted.
Dosimetry, Breast, Immobilization
TH- External Beam- Photons: Motion management - intrafraction